diff --git a/tractor/_actor.py b/tractor/_actor.py
index f1beb45..2604fa4 100644
--- a/tractor/_actor.py
+++ b/tractor/_actor.py
@@ -103,7 +103,7 @@ async def _invoke(
                 # TODO: we should really support a proper
                 # `StopAsyncIteration` system here for returning a final
                 # value if desired
-                await chan.send({'stop': None, 'cid': cid})
+                await chan.send({'stop': True, 'cid': cid})
             else:
                 if treat_as_gen:
                     await chan.send({'functype': 'asyncgen', 'cid': cid})
@@ -117,7 +117,7 @@ async def _invoke(
                     if not cs.cancelled_caught:
                         # task was not cancelled so we can instruct the
                         # far end async gen to tear down
-                        await chan.send({'stop': None, 'cid': cid})
+                        await chan.send({'stop': True, 'cid': cid})
                 else:
                     await chan.send({'functype': 'asyncfunction', 'cid': cid})
                     with trio.open_cancel_scope() as cs:
@@ -141,7 +141,7 @@ async def _invoke(
         tasks = actor._rpc_tasks.get(chan, None)
         if tasks:
             try:
-                tasks.remove((cs, func))
+                scope, func = tasks.pop(cid)
             except ValueError:
                 # If we're cancelled before the task returns then the
                 # cancel scope will not have been inserted yet
@@ -197,7 +197,7 @@ class Actor:
         self._no_more_rpc_tasks.set()
         self._rpc_tasks: Dict[
             Channel,
-            List[Tuple[trio._core._run.CancelScope, typing.Callable]]
+            Dict[str, Tuple[trio._core._run.CancelScope, typing.Callable]]
         ] = {}
         # map {uids -> {callids -> waiter queues}}
         self._actors2calls: Dict[Tuple[str, str], Dict[str, trio.Queue]] = {}
@@ -344,9 +344,10 @@ class Actor:
                 if msg is None:  # terminate sentinel
                     log.debug(
                         f"Cancelling all tasks for {chan} from {chan.uid}")
-                    for scope, func in self._rpc_tasks.pop(chan, ()):
+                    for cid, (scope, func) in self._rpc_tasks.pop(
+                        chan, {}
+                    ).items():
                         scope.cancel()
-
                     log.debug(
                             f"Msg loop signalled to terminate for"
                             f" {chan} from {chan.uid}")
@@ -354,10 +355,20 @@ class Actor:
                 log.debug(f"Received msg {msg} from {chan.uid}")
                 cid = msg.get('cid')
                 if cid:
-                    # deliver response to local caller/waiter
-                    await self._push_result(chan.uid, cid, msg)
-                    log.debug(
-                        f"Waiting on next msg for {chan} from {chan.uid}")
+                    cancel = msg.get('cancel')
+                    if cancel:
+                        # right now this is only implicitly used by
+                        # async generator IPC
+                        scope, func = self._rpc_tasks[chan][cid]
+                        log.debug(
+                            f"Received cancel request for task {cid}"
+                            f" from {chan.uid}")
+                        scope.cancel()
+                    else:
+                        # deliver response to local caller/waiter
+                        await self._push_result(chan.uid, cid, msg)
+                        log.debug(
+                            f"Waiting on next msg for {chan} from {chan.uid}")
                     continue
 
                 # process command request
@@ -403,7 +414,7 @@ class Actor:
                         log.info(f"RPC func is {func}")
                         # store cancel scope such that the rpc task can be
                         # cancelled gracefully if requested
-                        self._rpc_tasks.setdefault(chan, []).append((cs, func))
+                        self._rpc_tasks.setdefault(chan, {})[cid] = (cs, func)
                 log.debug(
                     f"Waiting on next msg for {chan} from {chan.uid}")
             else:
@@ -611,9 +622,9 @@ class Actor:
         """
         tasks = self._rpc_tasks
         log.info(f"Cancelling all {len(tasks)} rpc tasks:\n{tasks}")
-        for chan, scopes in tasks.items():
+        for chan, cids2scopes in tasks.items():
             log.debug(f"Cancelling all tasks for {chan.uid}")
-            for scope, func in scopes:
+            for cid, (scope, func) in cids2scopes.items():
                 log.debug(f"Cancelling task for {func}")
                 scope.cancel()
         if tasks: